The present invention relates to a precharge-type carry chained adder circuit well adaptable for high speed addition.
FIG. 1 shows a circuit diagram illustrating a one-stage part of a Manchester-type carry chained adder constructed with N type MOSFETs. The adder receives a precharge signal .phi., a carry kill signal K, a carry propagation signal P, and a carry in signal Carry-in, and produces carry propagation signals P' and P' and a carry-out signal Carry-out.
One of the most important problems involved in such an adder circuit is how to quickly send a carry signal to a more significant digit position. To cope with this problem, various types of high speed adders have been proposed. The approach to speeding up the carry signal reproduction, however, suffers from a large increase in additional hardware.
A known carry look-ahead circuit is advantageous in the reproduction speed of the carry signal, but is disadvantageous in the additional and complicated hardware. The carry look-ahead circuit further requires multi-input gates. The use of the multi-input gates is not recommended for the MOSFET circuit, particularly a CMOS circuit.
For these reasons, there is a great demand for realizing a high speed adder with a simple construction which can be adapted for the MOSFET circuit. One of the adder circuits thus far proposed to meet such a demand is a precharge type carry chained adder circuit. In this type of adder circuit, a carry is reproduced through N type transfer gates. With this arrangement, if each node in the transfer gate circuit is previously precharged to "H" a high level before the reproduction of a carry, a high speed reproduction of the carry is realized. In this arrangement, however, when no carry occurs, each node must be discharged through the gates connected in series between the carry-in and carry-out. In an extreme case, the gates corresponding in number to the word length of the adder must be connected in series. Therefore, as the word length of the adder is increased, the delay time resulting when the carry signal is reproduced is long and problematic.